The present invention relates to a method for producing a luminescent viewing screen in a cathode-ray tube (CRT) having a focus mask as a color selection electrode and, more particularly, to a method for making such a screen using a quadrupole focus mask as an optical pattern.
A conventional shadow-mask-type CRT comprises generally an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colorrs arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multiapertured sheet of metal precisely disposed between the screen and the beam-producing means. The apertured metal sheet shadows the screen, and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color.
In one prior art process for forming each array of phosphor elements on a viewing faceplate of a CRT, the inner surface of the faceplate is coated with a mixture of a photosensitive binder and phosphor particles adapted to emit light of one of the three emission colors. A light field is projected from a source through the apertures in the shadow mask and onto the coating so that the shadow mask functions as a photographic master. The exposed coating is subsequently developed to produce the first color-emitting phosphor elements. The process is repeated for the second and third color-emitting phosphor elements, utilizing the same shadow mask but repositioning the light source for each exposure. Each position of the light source approximates the convergence angle of one of the electron beams which excites the respective color-emitting phosphor elements. A more complete description of a prior art process for forming a viewing screen using a shadow mask can be found in U.S. Pat. No. 2,625,734 issued to H. B. Law on Jan. 20, 1953.
A drawback of conventional shadow-mask-type CRT's is that at about the center of the shadow mask all but about 18% of the beam currents is intercepted by the electron impervious portion of the shadow mask; that is, the shadow mask is said to have a transmission of about 18%. Thus, the area of the apertures of the shadow mask is about 18% of the area of the metal sheet. Since there are no focusing fields present in a shadow mask, a corresponding portion of the viewing screen is excited by the transmitted portions iof the electron beams.
Several methods have been suggested for increasing the transmission of the color selection electrode; that is, increasing the area of the apertures with respect to the area of the color selection electrode without substantially increasing the excited portions of the viewing screen area. In one approach, the apertures in the color selection electrode are enlarged and the transmitted portions of the electron beams are focused by magnetic or electric fields produced in the vicinity of each of the apertures. One such structure is known as a quadrupole focus mask. In a second approach, each aperture in the color selection electrode is enlarged and split into two adjacent windows by a conductor. The transmitted portions of the electron beams pass through the windows of each aperture, are deflected around the conductors towards one another, and fall onto substantially the same area of the viewing screen. Such a structure is known as a deflection-and-focus mask. A variation of this latter approach is described in U.S. Pat. No. 4,651,051 issued on Mar. 17, 1987 to Bloom et al., and entitled, CATHODE-RAY TUBE HAVING A FOCUSING COLOR-SELECTION STRUCTURE AND A VIEWING SCREEN FORMED THEREFROM. The deflection and focus mask color-selection structure described in the Bloom et al. patent can be used as a photographic master for producing the viewing screen; however, no quadrupole focus mask color-selection structure has yet been developed which can also serve as a suitable photographic master for producing a viewing screen. This is because the apertures in the quadrupole focus mask are substantially larger than the individual color-emitting phosphor screen elements. While it is known from U.S. Pat. No. 3,661,581 issued to Feldstein on May 9, 1972, to temporarily reduce the size of final size circular apertures in a conventional shadow mask, such an expedient is impractical for producing a line screen using substantially rectangular apertures, such as are found in the quadrupole focus mask.